To prepare cupcake, heat milk in microwave until just start boiling. Place tea bags in milk and let it steep for 30 minutes, and the mixture is at room temperature. Discard the tea bags. Reserve 3 tablespoons for buttercream frosting.

Preheat over to 350F. Grease muffin tin or line with muffin liners.

In a large bowl, sift flour, baking powder, baking soda and salt. Mix well and set aside.

Add sour cream and vanilla into the milk and tea mixture. Mix well and set aside.

In the bowl of an electric mixer fitted with the paddle attachment, mix canola oil and sugar on medium-high speed until well combined. Reduce the mixer speed to medium and slowly add eggs until just blended.

With the mixer on low, slowly add half the flour mixture; mix until just blended. Add the liquid mixture, mix until just blended. Slowly add remaining flour mixture, scraping down sides of the bowl with a spatula, as necessary, until just blended.

To prepare buttercream, beat butter for a few minutes in a standing mixer with the paddle attachment on medium speed. Add powdered sugar and turn your mixer on low, beat until the sugar incorporates with the butter. Add vanilla, heavy cream and the reserved Earl Grey tea, beat on medium speed for another 3 minutes, or until buttercream becomes light and fluffy.